Method of producing high strength sucker rod coupling

ABSTRACT

A sucker rod coupling 10 having a high ultimate tensile strength, resistance to corrosion, and resistance to surface cracking arising out of a method of making the sucker rod coupling 10, which employs a five-step process of forming a coupling 10. First, a hollow cylindrical core 12 from a heat treatable steel is formed. Second, a thin coating 18 of metallic alloy is applied to the outer surface of the core 12. Third, the core 12 is heat treated. Fourth, threads 20 are partially cut in the inner surface of the core 12. Fifth, the threads 20 are cold worked to transform the partially cut threads 20 into finished threads 20 and to place the thread roots 22 in compression.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to couplings used to connectpipe segments to form a string, and more particularly relates tocouplings adapted to connect sucker rods together to form a sucker rodstring for use in a producing well.

2. Description of the Related Art

Sucker rod couplings which connect individual sucker rods to form astring are a key component in the successful performance of a sucker rodstring. By minimizing the wear breakage and corrosion of couplings, thelife of the sucker rod string is increased, which can substantiallydecrease service and repair costs. However, sucker rod couplings can bethe weakest link in the sucker rod string, limiting the amount of stressthat the sucker rod string can absorb without failure.

The general industry practice has been to design sucker rod strings sothat stress range requirements of the particular well application arewithin the allowable limits of the sucker rod body. This practice,however, erroneously assumes that the sucker rod couplings will providethe same or greater service life. This assumption is not valid forcertain combinations of rods and couplings and results in derating theallowable coupling loads to give satisfactory coupling stress levels andcorresponding service life.

Derating refers to the determination of the ultimate tensile strength ofa particular coupling and sucker rod combination by multiplying theultimate tensile strength of the sucker rod by a suitable deratingfactor. The derating factor normally has a value less than 1 and variesdepending on the particular combination of coupling size and sucker rodsize, and material strength.

There are two common situations involving derating. The first caseinvolves the use of American Petroleum Institute (hereinafter "API")slimhole couplings with standard API or high strength grades of suckerrod. Although the ultimate tensile strength of the coupling and rod maybe similar, the reduced wall thickness of slimhole couplings reduces theeffective stress area thus creating an increase in stress in thecoupling for any given load. For example, a typical derating factor fora 1 inch (approximately 0.025 m) diameter API slimhole coupling, whenused with an API grade D sucker rod, is approximately 0.89. Thus, thecombination of a 1 inch (approximately 0,025 m) diameter API slimholecoupling with an API grade D rod can typically withstand only about 89%of the stress that the API grade D rod could sustain alone.

The second scenario involves the use of a full-size API coupling with ahigh strength sucker rod which typically has an ultimate tensilestrength in excess of 115,000 pounds per square inch (hereinafter"psi"). Here the ultimate tensile strength of the high strength suckerrod exceeds the ultimate tensile strength of the full-size API coupling.For example, a derating factor for a 7/8 inch nominal size(approximately 0.019 m) diameter full-size API coupling in combinationwith a high strength sucker rod is approximately 0.85. Thus, thecombination of a 7/8 inch (approximately 0.019 m) full-size API couplingwith a high strength sucker rod can withstand only about 85% of thestress that the high strength sucker rod could withstand alone.

Corrosive well environments further complicate the problem. Sour gaswells may cause sulfide cracking in coupling cores, or ordinarycorrosion may cause the cores to fail. A thin nickel based metallicalloy is typically applied to the outer surface of a sucker rod couplingto protect the coupling from corrosion and sulfide cracking. However,such coatings are themselves susceptible to stress cracking when appliedto coupling core materials that have a higher hardness and ultimatetensile strength, or when applied to a coupling core material that is tobe heat treated. The surface coating tends to become brittle during thesubsequent heat treatment.

Conventional fabrication techniques have failed to produce a sucker rodcoupling of sufficient fatigue strength, ultimate tensile strength, andresistance to corrosion and surface cracking to allow operators to fullyutilize the capability of existing sucker rods.

SUMMARY OF THE INVENTION

The present invention includes a new method for making a new sucker rodcoupling which can match the fatigue and ultimate tensile strength ofmodern sucker rods while providing a crack and corrosion resistantcoating. In a preferred embodiment, the high strength sucker rodcoupling includes a cylindrical core which has an ultimate tensilestrength in excess of about 146,000 psi and a Rockwell hardness(hereinafter HRC) in excess of about 32. A smooth coating surrounds theexterior of the cylindrical core. The coating has a hardness exceedingabout 45 HRC and is designed to resist corrosion and wear. Thecylindrical core also has a plurality of internal threads which aredesigned to engage the external threads of a typical sucker rod.

In another preferred embodiment adapted for mild well environments, thehigh strength sucker rod coupling includes a cylindrical core which hasan ultimate tensile strength in excess of about 146,000 psi and ahardness in excess of about 32 HRC. The cylindrical core also has aplurality of internal threads which are designed to engage the externalthreads of a typical sucker rod.

The present invention also includes a new method of producing a highstrength sucker rod coupling which entails forming or utilizing a hollowcylindrical core from a heat treatable steel, heating the core aboveabout 1800° F., and adding a thin layer of metallic alloy to the core.The alloy preferably contains nickel, chromium, silicon, and iron. Thecore is then cooled slowly to below about 150° F. The core is then heattreated, salt quenched, and air cooled. The core is then tempered andtapped. The final step involves cold working the core threads to placethe thread roots in a state of compression.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and reference to the drawingin which:

FIG. 1 depicts an exemplary high strength sucker rod coupling,illustrated in partial section.

While the invention is susceptible to various modifications andalternative forms, a specific embodiment thereof has been shown by wayof example in the drawing and will therein be described in detail. Itshould be understood, however, that this specification is not intendedto limit the particular form disclosed herein, but on the contrary, theinvention is to cover all modifications equivalents, and alternativesfalling within the spirit and scope of the invention, as defined by theappended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, therein is depicted an exemplary high strengthsucker rod coupling 10 in accordance with the present invention. Thecoupling 10 includes a core 12, end surfaces 14, 16, an external coating18, and threads 20.

In a preferred embodiment, the core 12 is drawn into a hollowedsubstantially cylindrical form. The core 12 is preferably fashioned fromAISI 4140 or 4142 steel, though any steel capable of obtaining requiredstrength/hardness through heat treatment may be suitable. Afterapplication of a suitable heat treatment, the core 12 preferably has aminimum ultimate tensile strength of approximately 117,000 psi and ahardness greater than about 23 HRC. It is especially preferred that thecore 12 has a minimum ultimate tensile strength of about 146,000 psi anda minimum hardness of between about 32 and about 36 HRC. The endsurfaces 14, 16 should be machined smooth by grinding or lapping ormachining to ensure that any preload applied to the coupling 10 and thesucker rods is retained.

The coating 18 is preferably a nickel based alloy applied to the core 12by a metal spray technique to be more fully disclosed below. In aparticularly preferred embodiment, the coating 18 is composed of aminimum thickness of 0.010 inches (approximately 0.00025 m) of Colmonoy#5 spray powder, which contains nickel, chromium, silicon, and iron.Other coatings may be suitable as well such as SCM 76-M-50 (M) metalpowder, or cobalt based powder which also contains iron, nickel, carbon,silicon, boron, chromium, and molybdenum. Other metal based powders, orother coatings, such as for example, ceramic or plastic coatings may besuitable as well, though plastic or other coatings which have arelatively low melting point will have to be applied after the core 12is heat treated. The temperature at which the coating 18 is fused to thecore 12 will depend upon the particular coating 18 material. The coating18 preferably has a minimum hardness of between about 45 and about 53HRC, but the hardness may be less than 45 HRC or exceed 53 HRC. Toensure that there is minimum friction and wear for both the coupling 10and the well tubing through which it is inserted, the coating 18 shouldbe ground to a smooth finish, or preferably, about 63 R_(a).

Since the core 12 has a much higher hardness than lower strength cores,formation of the threads 20 in the core 12 requires a slightly differentprocedure than that conventionally used. Ordinarily, the threads 20would be full-formed rolled in a cold working operation to place theroots 22 of the threads 20 in a state of compression and provideresistance to fatigue stress. However, owing to the hardness of the core12, in this application the threads 20 should first be partially cutwith an existing tap such that sufficient space remains in the threadroots 22 for metal displacement during a subsequent cold workingoperation. The threads 20, and particularly the thread roots 22, arethen cold worked using a cold form tap to place the thread roots 22 in astate of compression. Experimentation has shown that by using thistechnique of forming the threads 20, the effect of the cold workingextends to a depth of approximately 0.0030 inches (approximately0.000076 m), which is approximately 75% of the cold working depthachieved on a lower strength coupling wherein the threads 20 are formedby the conventional full-formed rolling process.

In another embodiment more suitable for mild well environments, thesucker rod coupling does not have an external coating. In all otheraspects, this embodiment of the sucker rod coupling is structurallyidentical to, and has the same physical properties as, the sucker rodcoupling 10 shown in FIG. 1.

A preferred method for fabricating the sucker rod coupling 10 shown inFIG. 1 includes application of a suitable corrosion resistant coating 18to the core 12, heat treating the coupling 10 to increase the ultimatetensile strength of the core 12 to above about 117,000 psi, andpreferably above about 146,000 psi, and the core 12 hardness preferablyabove about 23 HRC to between about 32 and about 36 HRC, and forming thethreads 20 by a combination of a partial tapping operation and a coldworking operation.

Since the coating 18 should be able to withstand the heat treatment ofthe core 12 after the coating 18 is applied to the core 12, the spraymetal technique for applying the coating 18 should be modified. Metalspray powder, preferably Colmonoy #5, is applied to the core 12 andfused between 1840° F. and 1860° F. The coating 18 formed thereby shouldhave a minimum thickness of about 0.010 inches (approximately 0.00025 m)and have a minimum hardness between about 45 and about 53 HRC. It ispreferred that immediately following application of the coating 18, thecoupling 10 be slow cooled to prevent the formation of cracks in thecoating 18. This is preferably done by immersing the coupling 10 in asuitable insulating material which will prevent rapid heat loss from thecoupling 10 by either conduction, convection, or radiation.Experimentation has shown that Vermiculite in about 1/4 inch(approximately 0.0064 m) granular size is particularly suitable as aninsulating material. The coupling 10 should be cooled in the Vermiculiteor other suitable material to below about 150° F. at a cooling rate notexceeding: about 41° F./min from about 1400° F. down to about 1200° F.;about 10° F./min from about 1200° F. to about 700° F.; and about 4°F./min from about 700° F. down to about 200° F., before removal from theVermiculite or other material. After the coupling 10 is slow cooled, itshould be checked for cracks in the coating 18 and the hardness of boththe core 12 and the coating 18 should be checked.

Following application of the coating 18 to the core 12, the coupling 10should be heat treated to increase the ultimate tensile strength of thecore 12. Aside from achieving high ultimate tensile strength, the goalof the heat treatment is to create in combination with martensite grainstructure with limited grain growth, such as bainitic in combinationwith martensite.

A preferable heat treatment, well known to those skilled in the art,comprises the following steps. The coupling 10 should be heated to about900° F. and held at that temperature for about thirty minutes. Thecoupling 10 is then raised to between about 1200° F. to 1225° F. andheld in that temperature range for about one hour. The coupling 10 isthen heated to about 1550° F., held at that temperature for about onehour and simultaneously exposed to a 0.40 carbon potential. The coupling10 is next quenched in salt at about 525° F. and held at thattemperature for about one hour. The coupling 10 is then air cooled tobelow about 150° F. and the core 12 hardness is again checked. The core12 should then be tempered to achieve a hardness of between about 32 andabout 36 HRC.

Following heat treatment, the threads 20 are formed in the core 12 by acombination of cutting and cold working. The threads 20 are firstpartially cut by a suitable tap which will leave space at the threadroots 22 for metal displacement during a subsequent cold workingoperation.

Threads may be formed in a conventional lower strength sucker rodcoupling by a pure full-formed rolling operation which does not removeany material from the coupling but rather displaces it, particularly atthe roots, thus increasing the fatigue strength at the roots. However,because the ultimate tensile strength of the core 12, in accordance withthe present invention, is substantially greater than about 100,000 psi,the core 12 is simply too hard for a pure cold working operation to formthe threads 20 therein.

After the threads 20 have been partially tapped, they should be coldworked using a cold form tap to displace, but not remove, some materialat the thread roots 22, with the goal of placing the thread roots 22 ina state of compression to give the thread roots 22 a heightenedresistance to fatigue stress cracking. For example, experimentation hasshown that a 1 inch nominal size (approximately 0,025 m) sucker rodcoupling 10 in accordance with the present invention has endured about107 cycles of a stress load alternating between about 13,006 psi toabout 54,904 psi without failure.

Subsequent to the cold working of the threads 20, the end surfaces 14,16 should be machined smooth and perpendicular to the threads 20 tominimize bending moment on the threads 20, and to preserve any preloadthat may be placed on the coupling 10 after rod insertion. In addition,the coating 18 should be ground to a smoothness of a 63 R_(a) finish tominimize friction and wear for both the coupling 10 and the particulartubing through which the coupling 10 is inserted.

For mild well environments, the above method need not include the spraymetal application. The strength and hardness of the core 12 and thethreads 20 may be increased using either the heat treatment andsubsequent threading operation disclosed above or by using a purefull-formed rolling operation with a core strength in excess of about 23HRC and at the required strength level, approximately 32 toapproximately 36 HRC.

Many modifications and variations may be made in the techniques andstructures described and illustrated herein without departing from thespirit and scope of the present invention. Accordingly, the techniquesand structures described and illustrated herein should be understood tobe illustrative only and not limiting upon the scope of the presentinvention.

We claim:
 1. A method of producing a sucker rod coupling, comprising the steps of:a. forming a hollow cylindrical core from a heat treatable steel, said core having an inner surface and an outer surface; b. applying a thin layer coating to said outer surface, said coating having a Rockwell hardness of at least 45 HRC; c. heat treating said core and said coating to elevate Rockwell hardness of said core to greater than 23 HRC; d. cutting partial threads in said inner surface of said core; said partial threads having roots; and e. cold working said partial threads to transform said partial threads into finished threads and to place said thread roots in compression.
 2. The method of claim 1 wherein said thin layer coating comprises an alloy containing nickel, chromium, silicon, and iron.
 3. The method of claim 1 wherein said thin layer coating comprises an alloy containing cobalt, iron, nickel, carbon, silicon, boron, chromium, and molybdenum.
 4. The method of claim 1 wherein said thin layer coating comprises a ceramic material.
 5. The method of claim 1 wherein said thin layer coating is a nickel based alloy containing chromium, silicon, and iron, and said step of applying said coating includes heating said core to above about 1800° F. then cooling said core to below about 200° F.
 6. The method of claim 1 wherein said thin layer coating is a nickel based alloy containing chromium, silicon, and iron and said step of heat treating said core includes:a. heating said core to about 900° F. for about thirty minutes; b. heating said core to about 1200° F. to about 1225° F. for about one hour; c. heating said core to about 1550° F. and subjecting said core to about 0.40 carbon potential for about one hour; d. quenching said core in salt at about 525° F., and maintaining said core temperature at about 525° F. for about one hour; e. cooling said core in air to below about 200° F. at a cooling rate not exceeding: about 41° F./min from about 1400° F. down to about 1200° F.; about 10° F./min from about 1200° F. down to about 700° F.; and about 4° F./min from about 700° F. down to about 200° F.; and f. tempering said core to obtain a Rockwell hardness of greater than 23 HRC at said inner surface of said core;
 7. A method of producing a sucker rod coupling, comprising the steps of:a. forming a hollow cylindrical core from a heat treatable steel, said core having an inner surface and an outer surface; b. heat treating said core to give said core an ultimate tensile strength greater than about 117,000 pounds per square inch; c. cutting partial threads in said inner surface of said core; said partial threads having roots; and d. cold working said partial threads to transform said partial threads into finished threads and to place said thread roots in compression.
 8. The method of claim 7 wherein said step of heat treating said core includes:a. heating said core to about 900° F. for about thirty minutes; b. heating said core to about 1200° F. to about 1225° F. for about one hour; c. heating said core to about 1550° F. and subjecting said core to about 0.40 carbon potential for about one hour; d. quenching said core in salt and about 525° F., and maintaining said core temperature at about 525° F. for about one hour; e. cooling said core in air to below about 200° F. at a cooling rate not exceeding: about 41° F./min from about 1400° F. down to about 1200° F.; about 10° F./min from about 1200° F. down to about 700° F.; and about 4° F./min from about 700° F. down to about 200° F.; and f. tempering said core to obtain a Rockwell hardness of at least 23 HRC at said inner surface of said core.
 9. A method of producing a sucker rod coupling, comprising the steps of:a. forming a hollow cylindrical core from a heat treatable steel, said core having an inner surface and an outer surface; b. heat treating said core to elevate the Rockwell hardness of said core to greater than 23 HRC; c. applying a thin layer coating of a plastic material to said outer surface after said core is heat treated; d. cutting partial threads in said inner surface of said core; said partial threads having roots; and e. cold working said partial threads to transform said partial threads into finished threads and to place said thread roots in compression. 